Concerning state of the art is known from the WO 95/27297, WO 95/27298, and U.S. Pat. No. 7,550,960 B2, where current and voltage sensor are integrated with embedded pole at line and load side.
With the known construction, the external surface of the coil is covered with semiconducting layer, which works as shielding for the internal coil at the same time works as the electrode of capacitive voltage sensor to high voltage primary, the problem is, that the voltage measurement accuracy of integrated current and/or voltage sensors will change with changing insulating resin surface conditions, for example as effected by external humidity, and the parasitic capacitance could change. This parasitic capacitance will also change the capacitance for measurement. Therefore the change of the measurement is shown in FIG. 3-4 as state of the art. The measurement capacitance C1 could have four components, C11 between the inner coil surface to primary conductor with high voltage (HV), left and right coil surface to primary conductor capacitance C12 and C13, and from the external surface C14 to primary conductor.